Leaf proline content and its relation to fruit load and flowering in citrus under field conditions

Citrus species grown in temperate zones have sprouting inhibited in winter and this is retaken in spring when they also bloom. The main factor that defines the vegetative or reproductive destiny of the buds is the presence of fruits in development. Low winter temperatures slow plant metabolism, which has been reported as necessary for the expression of the reproductive program. During the winter period of the annual cycle, osmotically active compounds such as proline are increased as part of the cold tolerance response. We investigated whether the presence of developing fruit and the application of exogenous gibberellins affect the proline level in citrus leaves. In an orchard of adult 'Montenegrina' mandarin trees, leaf proline contents were measured over two rest periods for branches with or without fruits. Branches that bloomed, the ones that did not have developing fruits, had higher proline levels as well as higher proline: chlorophyll and proline: total amino acid ratios than did branches with fruits. However, the application of exogenous gibberellins, which reduced flowering, did not affect proline content during the same periods. We discuss the ways in which proline may be involved in floral induction in citrus.